Security marks or watermarks are often made for purposes of authentication, brand protection, or production and protection, of valuable documents, such as bank notes, passports, drivers' licenses, authentication labels, tax stamps, and like documents to be secured. Watermarks are often used on such secured documents to provide an effective form of protection against counterfeiting. Watermark images on secured documents are typically invisible, or at least difficult to view, unless correctly back-illuminated from behind the documents.
There are several main categories of watermarks. A Fourdrinier or true watermark is made during paper manufacture. An image of the watermark is formed when different degrees of pressure are applied to the paper by a dandy roll, which contains the image, while the paper is still wet. The watermark effect is achieved by the paper being impressed, in selective areas, with a pattern of varying thicknesses, and by manipulating the amount or level of light that is transmitted through the varying thicknesses of the paper when illuminated from the back (transmitted back-light illumination). Thinner portions of the paper pass more light and appear to be lighter in color, while thicker portions of the paper absorb and block the transmitted light, and therefore appear to be darker in color. True watermarks are visible from either side of the paper when held up to the light (transmitted back-light illumination).
Another type of watermark is called the cylinder mold watermark. A shaded watermark, first used in 1848, incorporates tonal depth and creates a grey scale image. Instead of using a wire covering for the dandy roll, the shaded watermark is created by areas of relief on the roll's outer surface. Once dry, the paper may then be rolled, again to produce a watermark of even thickness, but with varying density. The resulting watermark is generally much clearer and more detailed than that made by the dandy roll process, and, as a result, cylinder mold watermark paper is the preferred type of watermarked paper for banknotes, passports, motor vehicle titles, and like documents where it is an important anti-counterfeiting measure.
An artificial, pseudo, or simulated watermark is typically created by printing an image in opaque white ink, transparent ink, or by using varnish. They will all produce an image that is visible only on the printed side of the paper and viewed at an angle illuminated by reflected light. Artificial watermarks are applied after the paper manufacturing process. They can be applied by the paper manufacturer, or by the printer. An artificial watermark can be seen from one side only, and is generally applied to the back side of the document, but can be applied to the front side as well.
Although the watermarking making methods described above are satisfactorily used with regular paper, i.e., cellulose pulp fibers pressed into a sheet, such methods have not been successfully applied to synthetic paper, i.e., hydrophilic, polymeric, micro-porous structures having a multitude of pores. Such synthetic paper is commonly used for secured documents, and is typically used where extraordinary durability, ease of printing and built-in security are critical.
Examples of hydrophilic, micro-porous, and polymeric structures, sheets, substrates, membranes or materials, which could be used as synthetic papers, include, but are not limited to, polyolefins, polyesters, polyvinyl halides, and acrylics with micro-voided structures. Preferred among these examples is a micro-porous substrate commercially available under the trademark “Teslin®” from PPG Industries, as defined in U.S. Pat. No. 4,833,172, the entire contents of which are hereby incorporated herein by reference thereto. A micrograph view of a Teslin® substrate, as seen by a scanning electron microscope, in accordance with the known art, is depicted in FIG. 5.
FIG. 1a depicts the structure of a micro-porous, polyolefin, structure made by a dry process in accordance with the known art, and FIG. 1b depicts the same micro-porous structure made by a wet process in accordance with the known art. As illustrated, each micro-porous structure is non-isotropic, that is, the pores or micro-void shapes and distributions of pores or micro-void sizes are not the same in planes perpendicular to the outer surface of the structure as compared to planes parallel to the outer surface of the structure. In the case of a Teslin® substrate, silica and the clays are the preferred siliceous fillers. Of the silica's, precipitated silica, silica gel, or fumed silica are most often used. In addition, minor amounts, usually less than about 5 percent by weight, of other materials used in processing, such as lubricant, plasticizer, processing plasticizer, organic extraction liquid, surfactant, water, and the like, may optionally also be present in the Teslin® substrate.
In the case of the Teslin® substrate, the micro-porous structure is stretched in at least one stretching direction. On an impregnant-free basis, pores constitute at least 50 percent, and preferably, up to more than 80 percent by volume of the micro-porous structure. In many cases, the pores constitute from about 60 to about 75 percent by volume of the precursor micro-porous structure. In all cases, the porosity of the micro-porous structure is, unless impregnated after stretching, greater than that of the precursor micro-porous structure. After stretching has been accomplished, the micro-porous structure may optionally be sintered, annealed, heat-set and/or otherwise heat-treated. During these optional steps, the stretched micro-porous structure is usually held under tension so that it will not markedly shrink at the elevated temperatures employed, although some relaxation amounting to a small fraction of the maximum stretch ratio is frequently permitted.
Following stretching and any heat treatments employed, tension is released from the stretched, micro-porous structure. After the micro-porous structure has been brought to a temperature at which, except for a small amount of elastic recovery amounting to a small fraction of the stretch ratio, it is essentially dimensionally stable in the absence of tension. Elastic recovery under these conditions usually does not amount to more than about 10 percent of the stretch ratio.
As shown in the known art of FIG. 2, the extinction of the light or luminous flux passing through a micro-porous structure occurs as a result of light scattering by the processes of reflection, refraction, interference and diffraction from the high degree of interconnectivity through the entire matrix of the structure. The scattering of light is due to gas-solid interfaces. The refractive index mismatch at the void/material interfaces is large, and, as a result, multiple scattering is unavoidable. This gives the bulk material its familiar white opaque appearance. In addition, some waveguide effect takes place within the interconnected membrane web. Light may be confined in the middle layer by total internal reflection. This occurs only if the dielectric index (refractive index) of the middle layer is larger than that of the surrounding layers. In addition, some light penetrates interconnected surface pores, bringing some luminous flux sub-surface. In the Teslin® substrate, which is highly impregnated with the aforementioned fillers, filler particles could act as a light barrier, or as a light reflector and diffuser.
As shown for a Teslin® substrate in the following Table 1, the amount of light transmission, i.e., the transmittivity or light transmission characteristic, of the substrate is correlated to the material thickness of the substrate. As an example, a 10 mils Teslin® substrate will transmit only 8 percent of the original light source lumens. Greater thicknesses will transmit less light, and vice versa.
TABLE 1Gauge (mils)5.7781010.512141418Gauge145178203254267305356356457(μm)Trans-171511896543mission(%)
Both the Fourdrinier watermark and the artificial watermark could be produced in micro-porous structures, sheets, substrates, materials and membranes. However, due to the fundamental differences between the structures of regular paper and synthetic paper, as described above, producing effective security marks on such micro-porous structures requires different methods. Accordingly, it would be desirable to reliably and effectively produce such security marks on such micro-porous structures.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The system and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.